the prop yes, as far as the dress goes, then that is fine too. Japanese are kind of embarassed and not too open with cosplay here as it is in the states..... Your practice is really up to you, sometimes i go to an event and my cosplay is in a really big bag, and sometimes its in a suitcase, it's just really up to you.

Also double check the event rules. TGS, for example, does not allow large props.

Whether or not you wish to be stealthy about your cosplay is up to you. Japanese people hide it because standing out in general is not a very Japanese thing to do! But if you don't care that people know you're on your way to go dress up, don't worry about trying to hide it.